The easiest way, indeed sometimes the only way to install a hump hose or replace an exhaust riser is to remove the exhaust flange from the engine at the manifold. Often the flange is so rusted it makes no sense to reinstall it.
The flange is part number 4 in the exploded view drawing found below in "More Images:".
Machined with a female 1-1/4" NPT for the matching male exhaust riser.
Applications include these engines (and the boats that we are aware used these engines... There could be other combinations as well):
• M12 (Catalina 25, Capri 26)
• M2-12 (Catalina 25, Capri 26)
• M18 (Catalina 27, Catalina 30)
• M25 (Catalina 30)
• M25XP (Catalina 28, Catalina 30, Catalina 34, Catalina 36)
• M3-20 (Catalina 27, Catalina 28, Catalina 30)
• M3-20B (Typically found in Catalinas used re-power)
• M35 (Catalina 34, Catalina 36, Catalina 38)
1) This flange is equipped with 1-1/4"NPT size threads and is not compatible with "B" series engines, which use 1-1/2"NPT size threads.
2) CAUTION: This part has been machined and then painted. There is some overspray in the fastener holes that must be cleared to accept engine studs.